1. Field of the Invention
The present invention relates to a modulating and demodulating device and, more particularly, to a modulating and demodulating device for use in a transmitter-receiver which comprises a modulator-demodulator (circuit) for performing a modulating or demodulating operation in accordance with a bias voltage to be supplied to diodes, transmits a signal at the time of the modulating operation and receives a signal at the time of the demodulating operation.
2. Related Art
At present, an automatic toll collecting (ATC) system has been being developed. According to the system, a roadside device is arranged on a toll facility side as means for collecting a toll of a toll road, an on-vehicle device is mounted on a vehicle, required radio waves are transmitted and received between the roadside device and the on-vehicle device when the vehicle stops at the toll facility, and a toll is collected by wireless means. Various tests are being carried out in order to put the system to practical use.
FIG. 3 is a block diagram showing a construction of an important part of a roadside device for use in the automatic toll collecting system. FIG. 4 is a block diagram showing a construction of an important part of an on-vehicle device for use in the automatic toll collecting system.
FIG. 5 is a circuit construction diagram showing a specific example of a construction of the on-vehicle device shown in FIG. 4. In FIG. 5, the same component elements as those shown in FIG. 4 are designated by the same reference numerals.
As shown in FIG. 3, the roadside device arranged on the toll facility side comprises an RF receiving filter 31, an RF receiving amplifier 32, a frequency converter 33, an IF (intermediate frequency) amplifier 34, an IF (intermediate frequency) filter 35, a local oscillator 36, first and second buffer amplifiers 37 and 38, a switch 39, an FM encoder 40, an RF transmitting amplifier 41, an RF transmitting filter 42, a receiving antenna 43, a transmitting antenna 44, a demodulation signal output terminal 45, and a modulation signal input terminal 46.
In this case, in the RF receiving filter 31, the input terminal is connected to the receiving antenna 43 and the output terminal is connected to the input terminal of the RF receiving amplifier 32. In the frequency converter 33, a first input terminal is connected to the output terminal of the RF receiving amplifier 32, a second input terminal is connected to the output terminal of the first buffer amplifier 37, and the output terminal is connected to the input terminal of the IF amplifier 34. In the IF filter 35, the input terminal is connected to the output terminal of the IF amplifier 34 and the output terminal is connected to the demodulation signal output terminal 45. The local oscillator 36 includes an oscillator, a phase locked loop, a quartz oscillator, a frequency multiplying circuit, and an output buffer amplifier (all of which are not shown in the diagram). The output terminal of the local oscillator 36 is connected to both the input terminal of the first buffer amplifier 37 and the input terminal of the second buffer amplifier 38. In the switch 39, the input terminal is connected to the output terminal of the second buffer amplifier 38, the output terminal is connected to the input terminal of the RF transmitting amplifier 41, and the control terminal is connected to the output terminal of the FM encoder 40. The input terminal of the FM encoder 40 is connected to the modulation signal input terminal 46. The input terminal of the RF transmitting filter 42 is connected to the output terminal of the RF transmitting amplifier 41 and the output terminal is connected to the transmitting antenna 44.
As shown in FIG. 4, the on-vehicle device to be mounted on a vehicle comprises a microstrip line (MSL) network 47, amodulator-demodulator 48, a diode 48D, abias voltage supplying circuit 49, an FM decoder 50, a transmitting/receiving antenna 51, a demodulation signal output terminal 52, and a change-over signal supplying terminal 53.
In this case, the input terminal of the microstrip line network 47 is connected to the transmitting/receiving antenna 51 and the output terminal is connected to the input terminal of the modulator-demodulator 48. In the modulator-demodulator 48, the diode 48D is serially connected between the input and output terminals, the output terminal is connected to the input terminal of the FM decoder 50, and the control terminal is connected to the output terminal of the bias voltage supplying circuit 49. The input terminal of the bias voltage supplying circuit 49 is connected to the change-over signal supplying terminal 53. The output terminal of the FM decoder 50 is connected to the demodulation signal output terminal 52.
Further, as shown in FIG. 5, the microstrip line network 47 has first to third microstrip lines 47A, 47B, and 47C. The modulator-demodulator 48 has, besides the diode 48D, a first inductor 48A, a second inductor 48C, a resistor 48C, and a bias voltage supplying terminal 48E. The bias voltage supplying circuit 49 has a switch 49A and a power source 49B. In FIG. 5, a control unit 54 which is not shown in FIG. 4 is provided.
In the microstrip line network 47, the first microstrip line 47A is serially connected between the input and output terminals, the secondmicrostrip line 47B is connected between the input terminal and the reference potential (ground) point, and the third microstrip line 47C is connected between the output terminal and the reference potential (ground) point. In the modulator-demodulator 48, the diode 48D is serially connected between the input and output terminals, the first inductor 48A and the resistor 48C are serially connected between the input terminal and the bias voltage supplying terminal 48E, and the second inductor 48C is connected between the output terminal and the ground terminal. In the bias voltage supplying circuit 49, one terminal of the switch 49A is connected to the output terminal of the bias voltage supplying circuit 49, another terminal is connected to the ground terminal via the power source 49B, and the control terminal is connected to the change-over signal supplying terminal 53. The control unit 54 is connected to the FM decoder 50 and the change-over signal supplying terminal 53.
Modes of transmitting and receiving signal waves between the roadside device and the on-vehicle device in the automatic toll collecting system having the above construction will be generally described as follows with reference to FIGS. 3 to 5.
First, a first operating mode of transmitting signal waves from the roadside device side to the on-vehicle device side will be described.
In the first operating mode, on the roadside device side, the FM encoder 40 operates in response to a modulation signal supplied to the modulation signal input terminal 46, and the switch 39 is turned on and off according to a signal encoded by the FM encoder 40. At this time, a carrier signal generated from the local oscillator 36 is subjected to amplitude shift keying (ASK) modulation by the on-off control by the switch 39, thereby forming an ASK modulated signal. Subsequently, after the ASK modulated signal is amplified by the RF transmitting amplifier 41 to a required level, unnecessary frequency components are eliminated by the RF transmitting filter 42, and the resultant signal is transmitted from the transmitting antenna 44.
In the first operating mode, the on-vehicle device receives the ASK modulated signal transmitted from the roadside device. An un-modulated signal is supplied from the control unit 54 to the change-over signal supplying terminal 53. By the supply of the un-modulated signal, the switch 49A of the bias voltage supplying circuit 49 is turned off, an output of the power source 49B is cut off by the switch 49A, the bias voltage supplying terminal 48E in the modulator-demodulator 48 is zero biased, and the diode 48D in the modulator-demodulator 48 is also zero biased. At this moment, when the signal waves from the roadside device are received by the transmitting/receiving antenna 51, the reception signal is supplied to the modulator-demodulator 48 via the microstrip line network 47, the ASK modulated signal is demodulated by the diode 48D, and an encoded signal led from the modulator-demodulator 48 is supplied to the FM decoder 50. The FM decoder 50 decodes the encoded signal, reproduces the original signal, and the resultant signal is supplied from the demodulation signal output terminal 52 to an adopted circuit (not shown).
A second operating mode of transmitting signal waves from the roadside device side to the on-vehicle device side and, in response to the transmission, transmitting signal waves from the on-vehicle device side to the roadside device side will now be described.
In the second operating mode, on the roadside device side, the FM encoder 40 generates an encoded signal having a unipolar pulse in response to an un-modulated signal supplied to the modulation signal input terminal 46 to maintain the switch 39 in the ON state. At this time, a carrier signal generated from the local oscillator 36 is supplied to the RF transmitting amplifier 41 through the switch 39 and is amplified to a required level by the RF transmitting amplifier 41. After that, unnecessary frequency components are eliminated by the RF transmitting filter 42, and a resultant signal is transmitted from the transmitting antenna 44.
In the second operating mode, the on-vehicle device receives the carrier signal transmitted from the roadside device and transmits a PSK modulated signal obtained by executing the phase shift keying (PSK) modulation to the carrier signal to the roadside device side. A binary modulation signal is supplied from the control unit 54 to the change-over signal supplying terminal 53 and the switch 49A in the bias voltage supplying circuit 49 is turned on or off in accordance with the supply of the binary modulation signal. When the switch 49A is ON, an output voltage of, for example, 5V of the power source 49B is supplied to the bias voltage supplying terminal 48E in the modulator-demodulator 48 and a forward voltage of 5V is applied to the diode 48D in the modulator-demodulator 48. On the other hand, when the switch 49A is OFF, the output voltage of the power source 49B is cut off by the switch 49A and the diode 48D is zero biased. At this moment, when the signal waves including the un-modulated carrier signal from the roadside device are received by the transmitting/receiving antenna 51, the reception signal is supplied to the modulator-demodulator 48 via the microstrip line network 47. The modulator-demodulator 48 performs a phase shift of 180.degree. between the phase of the un-modulated carrier signal when the forward voltage of 5V is applied to the diode 48D and that of the un-modulated carrier signal when zero bias is applied to the diode 48D, that is, executes the PSK modulation. After that, the obtained PSK modulated signal is transferred to the transmitting/receiving antenna 51 via the microstrip line network 47 and is transmitted from the transmitting/receiving antenna 51.
In the second operating mode, when the signal waves including the PSK modulated signal transmitted from the on-vehicle device side are received by the receiving antenna 43 on the roadside device side, after unnecessary frequency components in the reception signal are eliminated by the RF receiving filter 31, the resultant signal is amplified by the RF receiving amplifier 32 to a required level, and the amplified signal is supplied to the frequency converter 33. The frequency converter 33 frequency-mixes the reception signal with a local oscillation signal supplied from the local oscillator 36, thereby generating an intermediate frequency signal. The intermediate frequency signal is amplified to a required level by the IF amplifier 34. After that, unnecessary frequency components are eliminated by the IF filter 35 and a resultant signal is supplied to an adopted circuit (not shown) including a PSK modulation signal demodulator via the demodulation signal output terminal 45.
According to the modulating and demodulating device comprising the microstrip line network 47, the modulator-demodulator 48, and the bias voltage supplying circuit 49 arranged on the on-vehicle device side, in the first operating mode, when the diode 48D in the modulator-demodulator 48 is zero biased and a reception signal including the ASK modulated signal received by the transmitting/receiving antenna 51 is ASK demodulated by the diode 48D in the modulator-demodulator 48, the level of the demodulated signal is lower than the level of the reception signal (ASK modulated signal) received by the transmitting/receiving antenna 51 by 3 dB, that is, the half of the electric power of the reception signal is consumed at the time of ASK demodulation. Consequently, there is a problem such that the reception signal cannot be ASK demodulated with high efficiency.
In the second operating mode, the modulating and demodulating device disposed on the on-vehicle device side turns on or off the diode 48D by a modulation signal, executes the PSK modulation by performing a phase shift of 180.degree. between the phase of the carrier signal received by the transmitting/receiving antenna 51 when the diode 48D is ON and that of the carrier signal when the diode 48D is OFF. When the obtained PSK modulated signal is reflected by the modulator-demodulator 48 and is transmitted from the transmitting/receiving antenna 51, the level of the PSK modulated signal reflected by the modulator-demodulator 48 is lower than the level of the reception signal by 3 dB, that is, the half of the electric power of the reception signal is consumed at the time of the PSK modulation. Consequently, there is a problem such that the reception signal cannot be PSK modulated with high efficiency.